1. Field of the Invention
The invention relates to a rack and pinion type electric power steering system.
2. Description of Related Art
Conventionally, there is an electric power steering system (EPS) in which a motor is provided as a drive source and a steering sensor is provided to detect a steering angle of a steering wheel as an absolute angle in a range exceeding 360° (for example, Japanese Patent Application Publication No. 2012-21890 (JP 2012-21890 A)). The steering sensor generally has a low resolution as compared to a relative angle sensor such as a resolver that detects a rotation angle of the motor (a motor angle) as a relative angle in a range of 360°. Thus, the steering angle may be detected as an absolute angle, based on a motor angle detection value detected by a relative angle sensor by obtaining a value of the motor angle at a steering neutral position (a motor neutral point) based on a steering angle detection value detected by the steering sensor (for example, Japanese Patent Application Publication No. 2010-162954 (JP 2010-162954 A) and Japanese Patent Application Publication No. 2012-025262 (JP 2012-025262 A)).
Specifically, the motor neutral point is computed by subtracting a conversion value from the motor angle detection value detected by the relative angle sensor. The conversion value is obtained by multiplying the steering angle detection value (a value with respect to the neutral position) detected by the steering sensor, by a constant coefficient indicating a ratio of a rotation amount of the motor with respect to a rotation amount of the steering shaft. The steering angle based on the motor angle detection value is used for, for example, performing steering return control that causes the steering wheel to return to the neutral position (for example, Japanese Patent Application Publication No. 2008-168659 (JP 2008-168659 A)).
There is an EPS that includes a steering force assist device that applies an assist force to a steering system by converting the rotation of the motor to reciprocating motion of the rack shaft through a conversion mechanism other than a rack and pinion mechanism to which the steering shaft is connected. In recent years, for the purpose of, for example, improving steering feel, an EPS with a so-called variable gear ratio has been suggested, in which specifications of rack teeth constituting the rack and pinion mechanism vary depending on a position in a rack shaft in an axial direction so that a steering gear ratio is changed depending on the steering angle (for example, Japanese Patent Application Publication No. 2004-182138 (JP 2004-182138 A)).
In a case where a variable gear ratio is employed in the EPS including the steering force assist device that includes a conversion mechanism other than the rack and pinion mechanism to which the steering shaft is connected, when the motor neutral point is obtained by the conventional method described above, the motor angle detection value detected at the time when steering is actually performed to the neutral position may be a value deviated from the motor neutral point, depending on the steering angle at this time. That is, there is a possibility that the steering angle based on the motor angle detection value may be deviated from the actual steering angle, depending on the steering angle at the time when the motor neutral point is obtained.